1. Field of the Invention
This invention relates to the fluid catalytic cracking of hydrocarbons and is specifically concerned with the separation of the catalysts from the resultant cracked stream.
2. Description of the Prior Art
The fluid catalytic cracking process, with the advent of the highly active zeolitic type catalysts, has evolved into the generally standardized practice of effecting essentially cocurrent ascending flow of hydrocarbon vapors and the finally divided catalyst in an elongated tubular reaction zone referred to in the industry as a riser tube. Notwithstanding the brevity of of the reaction cycle, which is usually in the order of about 10 seconds or less, there is a laying down of coke on the cataylsts thereby adversely affecting its activity as well as undesirably altering product yield distribution upon reuse. Accordingly, it is the universally observed procedure to separate the catalyst from the riser tube effluent and recycle to the process via a regenerator which serves the dual purpose of combusting the coke contaminants and heating the catalyst for reuse in the reaction cycle.
In accordance with the prior art, separation of the catalyst from the cracked hydrocarbons is carried out within a so-called disengaging chamber. The disengaging chamber is a contained vessel either forming a relatively voluminous shroud about the downstream extremity portion of the riser tube or externally positioned and axially aligned therewith. In the latter type arrangement the fluid stream of catalyst and converted hydrocarbons is discharged into the disengaging chamber directly from the riser tube via a sidewise opening or port. In said shroud type arrangement, however, it is generally preferred to pass the riser tube effluent from said sidewise opening or port firstly through a single-stage cyclone vented to the disengaging chamber. Due to the substantial reduction of the superficial space velocity experienced in the disengaging chamber in either of said modes of operation, a considerable portion of catalyst entrained in the riser tube effluent settles out and collects at the bottom of the chamber. The gas stream thereupon is vented to the fractionator from disengaging chamber via a cyclonic separator positioned therein serving to collect predominantly all of the entrained catalyst.
In light of the fact that the modern zeolitic cracking catalysts range in particle size from about 5 to 100.mu. with the major portion thereof being in the order of from about 40 to 80.mu., separation thereof as practiced in accordance with the aforesaid prior art is nonetheless remarkably efficient. However, the relatively small amount of catalyst entrained in the cracked stream poses a problem because of the tremendous cumulative throughput thereof. The foremost problem is that the catalyst leaving the cracking unit must be recycled thereto in the form of a slurry oil recovered from the fractionator thereby reducing the amount of feedstock that can be accommodated in the cracking unit. Thus, optimal processing efficiency of the cracker is sacrificed.
Accordingly, the principal object of this invention is to secure a more rapid as well as a more efficient separation of the catalyst from the effluent of the riser tube without requiring any basic alteration of the design structure of the recovery systems currently used for this purpose.